Obviously that at some past time neither existed.
a) Wouldn't this third option require proving not one but TWO negatives?
b) The God I speak of is the creator of everything, including time itself, to which he would not be subject. He lives in an eternal present that knows no passage of time any more than it knows the limitations of the 3 dimensions of space. This God by definition would never have not existed -- there would not be any pre-God or post-God -- so you must be referring to a different god, a sort of naturalistic god that emerged from the universe instead of being its creator.